The present invention relates to the production Process for thermo-expansive microcapsules, more precisely, thermo-expansive microcapsules that disperse well in a medium such as paint or ink.
The production process of thermo-expansive microcapsules wherein a thermoplastic polymer microencapsulates a volatile blowing agent gasifying below the softening point of the thermoplastic polymer and forms microcapsules has been studied. Japanese Patent Publication Sho 42-26524 describes a production process that covers general thermo-expansive microcapsules and U.S. Pat. No. 3615972 describes the production process for thermo-expansive microcapsules having a polymer shell of uniform thickness. And Japanese Patent No. 2894990 describes the production process for heat resistance and thermo-expansive microcapsules consisting 80% or more of acrylonitrile monomer.
In those known processes, thermo-expansive microcapsules are produced in suspension-polymerization wherein an inorganic additive such as colloidal silica is employed as a dispersing and stabilizing agent and an organic additive is employed as a stabilizing auxiliary.
Those known thermo-expansive microcapsules do not disperse well in a medium such as polymers, paints and inks, in other words, it is difficult to disperse the microcapsules uniformly in a medium. The cause of the problem is estimated to be inorganic additives used as a dispersant and remaining on microcapsule surface.
On the other hand, Japanese Patent No. 2584376 describes a production process for expansive thermo-plastic fine particles wherein a powder stabilizer comprising the metal salt or hydroxide of one of Ca, Mg, Ba, Fe, Zn, Ni and Mn is employed. In this process an acid must be added after polymerization for the purpose of removing the powder stabilizer from the surface of fine particles. And it is not easy to produce fine particles as aimed. In addition, it is difficult to produce fine particles constantly in this process because of difficult pH control.
Japanese Patent Laid Open Publication Hei 11-209504 describes a process wherein expansive particles that expand quickly and uniformly are produced through suspension polymerization of polymerizing mixture without polymer coagulation by employing one or more of the compounds selected from the group comprising alkali metal nitrites, stannous chloride, stannic chloride, water-soluble ascorbic acids and boric acid. Even in this process inorganic additives and powder stabilizers cannot easily be removed from resultant particles and the particles do not disperse well in various media.
The object of the present invention is to solve the above problems and to provide the production process of thermo-expansive microcapsules which disperse better than conventional products.
Other objects and advantage of the present invention are shown in the following description.
The above objects and advantage of the present invention are attained by the production process of thermo-expansive microcapsules wherein a polymer is produced by polymerizing unsaturated ethylene monomers and the polymer microencapsulates a volatile blowing agent having a gasifying point lower than the softening point of the polymer and forms thermo-expansive microcapsules. Said process is characterized by the the polymer produced through the suspension-polymerization of unsaturated ethylene monomers under the existence of one or more of metal compounds selected from the group comprising zirconium compounds and titanium compounds.
The preferable zirconium and titanium compounds which sufficiently function in the process of the present invention are metal compounds such as zirconium sulfate, zirconium acetate, zirconium chloride, zirconium oxide, zirconium nitrate, titanium chloride, titanium oxide and titanium sulfate. Among those, zirconium sulfate and titanium chloride are more preferable because they are available in the form of aqueous solution that can be added to the water phase in the process with less labor. The above metal compounds bring a new advantage different from those of the conventional processes. In the conventional processes, inorganic additives functioning as a dispersing and stabilizing agent are incorporated in the wall of microcapsules during polymerization and cannot be removed easily. Such additives included in the wall of microcapsules inhibit the dispersion of such microcapsules in various media. In the process of the present invention, inorganic additives, are not incorporated in the wall of microcapsules and thus no labor for removing the additives is necessary. In addition, the additives function as a superior dispersing and stabilizing agent that can attain constant polymerization. Thus the thermo-expansive microcapsules produced in the process have superior property on their surface and disperse well in various media such as polymers, paints and inks.
The process for producing the thermo-expansive microcapsules of the present invention is similar to the conventional processes except employing the above-mentioned metal compounds. In the process, polymerizing monomers and a cross-linking agent are blended with a volatile blowing agent and an initiator and the mixture is processed in suspension-polymerization in an aqueous medium containing a proper quantity of an emulsifier and dispersant. The polymerizing monomers, cross-linking agent, initiator, volatile blowing agent, dispersing and stabilizing agent and other agents are not restricted within a certain range but are those already known.
The examples of the unsaturated ethylene monomers applied in the present invention are nitrile monomers, methacrylate, acrylate, vinylidene chloride, vinyl chloride, styrene, vinyl acetate and butadiene. One of those monomers or the combination of two or more of those monomers can be used. The combination can be determined according to the softening point, thermo-durability, durability against chemicals and end use of resultant polymer. For example, copolymers containing vinylidene chloride or nitrile monomers have superior gas-shielding function and copolymers containing 80 weight percent or more of nitrile monomers have superior thermo-durability and durability against chemicals.
Cross-linking agents can be added to unsaturated ethylene monomers if necessary. The examples of such cross-linking agents are divinyl benzene, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, triacryl formal, trimethylol propane tri-methacrylate, aryl methacrylate, 1,3-butyl glycol dimethacrylate and triaryl isocyanate.
The polymer which forms the wall of microcapsules is prepared by adding polymerization initiator to the above components and polymerizing them. Known initiators, such as peroxides and azo compounds, can be used. Azobis isobutylonitrile, benzoyl peroxide, lauryl peroxide, diisopropyl peroxydicarbonate, t-butyl peroxide and 2,2xe2x80x2-azobis (2,4-dimetyl) valeronitrile are the examples of such initiators. Oil-soluble initiators which are soluble in the monomers used for polymerization are preferable.
Volatile blowing agents encapsulated into microcapsules are known agents, which gasify below the softening point of capsule-wall polymers. Examples of those agents are low-boiling-point liquid including propane, propylene, butene, normal butane, isobutane, isopentane, neopentane, normal pentane, hexane, heptane, petroleum ether, halogen compounds of methane and tetra-alkyl silane or compounds gasifying through thermal degradation under heating, such as AIBN. A preferable blowing agent is one of or a mixture of two or more of low-boiling-point liquid, such as isobutane, normal butane, normal pentane and isopentane.
The aqueous medium in which suspension-polymerization is carried out can be prepared by adding a dispersing and stabilizing agent in deionized water and stabilizing auxiliaries are added if necessary. The examples of the dispersing and stabilizing agents are silica, calcium phosphate, calcium carboxylate, sodium chloride and sodium sulfate. The examples of the stabilizing auxiliaries used with the said dispersing and stabilizing agent are di-ethanol amine-adipic acid condensates, gelatin, methyl cellulose, polyvinyl alcohol, polyethylene oxide, dioctyl sulfosuccinate, sorbitane ester and emulsifiers. The polymerization of the present invention is carried out by adding the above-mentioned metal compound in the aqueous medium and by controlling the pH of the mixture approximately. 1 to 4, preferably 2 to 3.